The invention concerns a tensioning device for a traction element of a drive arrangement, said tensioning device comprising a tensioner-damper unit, a first tension element in contact with the traction element and a second tension element in contact with the traction element, with following features:
the tensioner-damper unit acts between a first pivoting lever and a second pivoting lever,
the first tension element is connected to the first pivoting lever,
the first pivoting lever is mounted on a fulcrum for pivoting about a first fixed axis of pivot,
the first tension element is urged by the first pivoting lever, under action of the tensioning force of the tensioner-damper unit, against a first strand,
the second tension element is connected to the second pivoting lever,
the second pivoting lever is mounted on a fulcrum for pivoting about a second fixed axis of pivot,
the second tension element is urged by the second pivoting lever, under action of the tensioning force of the tensioner-damper unit, against a second strand,
a deflector pulley for the traction element is arranged between the first strand and the second strand and
the first strand and the second strand are subjected to unequal tractive forces.
A tensioning device of the pre-cited type is known from DE 42 43 451 A1. This tensioning device is made up of two pivoting levers each of which has a tension element, a tensioner and a damping element arranged between the pivoting levers. The tension elements are preferably configured as tension rollers. The pivoting levers are fixed at one end on an axis of pivot for pivoting relative to each other. A tension roller is rotatably fixed on the other ends of the pivoting levers. Under the action of the tensioning force of the tensioner, the pivoting levers are urged by their tension rollers against the traction element, one of the pivoting levers acting on a slack strand and the other of the pivoting levers acting on a tight strand. A damping element acting equally on both pivoting levers is arranged between the pivoting levers. Through the tractive or compressive force of its spring system, the tensioner acts simultaneously on both pivoting levers. The tensioner and the damper are preferably combined into a tensioner-damper unit in a single assembly.
The term xe2x80x9cstrandxe2x80x9d designates that part of a traction element that is situated between two adjacent deflector pulleys of a drive arrangement. The deflector pulley can act:
as a pure deflecting device for the traction element, in which case, no or only an insignificant amount of power is drawn from the drive arrangement by the deflector pulley,
as a deflecting device and simultaneously as a driven pulley, for example as a pulley of a generator, in which case, power is drawn from the driving arrangement,
as a deflector pulley and simultaneously as a driving pulley, for example as a belt pulley of a crankshaft, in which case, power is fed to the drive arrangement.
In a drive arrangement, the slack strand is that part of the traction element that follows after a driving pulley in the direction of rotation and thus in the driving direction, while the tight strand is the entrained part of the traction element which follows directly after the driving pulley in the direction opposite to the driving direction, and/or is arranged between two driven pulleys in this direction. A slack strand is thus subjected to other tractive forces than a tight strand i.e., the tractive forces in the slack strand are lower than those in a tight strand.
The function of such a tensioner-damper unit is to compensate for length variations of the traction element that occur during or in the course of its operation and act particularly in the slack strand, and also to offset the consequences of torsional vibrations of the driving pulley, for example, of the belt pulley of a crankshaft. Length variations of the traction element are compensated for in order to achieve a uniform wrap of the traction element on the deflector pulleys in all conditions of operation during the entire operative life of the driving arrangement. Vibrations are compensated for by the damping element to keep the driven pulleys and the aggregate units that are connected thereto free of cyclic irregularities. By a uniform i.e., slip-free wrap on the driven pulleys, the driving power is transmitted with insignificant losses. At the same time, the traction element is subjected to high loads. The aim in view is to adapt the tensioning force applied to the strand by the tensioner to the power requirement or the power output of the associated deflector pulley. This means that, when the requirement of power is low, the traction element should also be pre-tensioned only with a low tensioning force. At a higher power flow, the tensioning force must also be higher. Therefore, ideally, the level of loading of the traction element should only be as high as required by the actual power flow.
It is difficult to achieve an ideal loading of the traction element when the tensioning device has to produce different tensioning forces for each of the strands to be tensioned for achieving an ideal loading of the traction element. This is the case, for example, when the tensioning device is to be used between a strand arranged on the left of a deflector pulley and a strand arranged on the right of the deflector pulley, and the left-hand strand, depending on the operative state of the drive arrangement, is, at one time, a tight strand while the right-hand strand is a slack strand, and at another time, the left-hand strand becomes a slack strand while the right-hand strand becomes a tight strand. An example of this are the strands on the left and the right of a belt pulley of a starter-generator unit that are integrated in a belt drive of an internal combustion engine. When the generator works as a starter, the strand between the belt pulley of the starter-generator unit and the belt pulley of the crankshaft is a tight strand and the strand on the other side of the belt pulley of the starter-generator unit is a slack strand. When the internal combustion engine has been started, the belt is entrained by the belt pulley of the crankshaft in the same direction of rotation and drive and the strand between the belt pulley of the starter-generator unit and the belt pulley of the crankshaft is a slack strand, while the strand on the other side of the belt pulley of the starter-generator unit is a tight strand.
During the starting operation, the forces in the tight strand between the belt pulley of the starter-generator unit and the belt pulley of the crankshaft are very high because when the internal combustion engine is being started by the starter-generator unit, the moments of drag for entraining the crankshaft are very high. The forces of the strand on the other side of the belt pulley of the starter-generator unit are not so high when this strand is a tight strand because, in this case, the aggregate units are entrained by the driving crankshaft. If, as mentioned above, the belt is to be pre-tensioned ideally only in correspondence to the power flow and thus in correspondence to the effective forces, the tensioner-damper unit must apply different forces to each strand depending on the aforesaid operative states of the internal combustion engine.
A drawback of the prior art tensioning device or tensioner-damper unit when used in an arrangement of the type described above is that the distribution of tensioning force among the deflector pulleys is not satisfactory. For pre-tensioning the left and right strands in keeping with their loading, the prior art requires for each strand, a tensioner specially matched to its particular loading. A prior art traction element drive of this type is described in U.S. Pat. No. 2,954,726. The traction element in this drive arrangement is a chain. The drive arrangement can be operated selectively in clockwise or anti-clockwise direction. Each of the opposite strands is loaded by a lever acting through a tension roller. Each lever is actuated by a spring system that is independent of the spring system acting through the other lever on the other strand. Such a solution does not permit any power-dependent control of the spring-damper characteristic and, due to double the number of individual components, disadvantageously affects the overall costs of such a drive arrangement.
The object of the invention is therefore to create a tensioning device
that acts on two strands that have different tractive forces,
that pre-tensions the strands by only one tensioner and
in which the tension forces are adapted to the differing tractive forces.
This object is achieved according to the characterizing part of claim 1 in that the pivoting levers of the tensioning device are formed by lever arms, one inner lever arm and one outer lever arm, that extend from the fulcrum of each pivoting lever. The free end of the outer lever arm of each pivoting lever is connected to a tension element, preferably a tension roller. The free end of the inner lever arm of each pivoting lever is articulated on one end of the tensioner-damper unit. The tensioner-damper unit is not stationary i.e., it is only secured between the inner lever arms and transmits, with a damping effect, the reaction forces occurring during the tensioning operation between the first and the second strand. The lengths of the inner and the outer lever arms of each pivoting lever are configured in a defined ratio to each other. This length ratio is different for each of the pivoting levers. It is matched to the required tensioning forces, that differ from strand to strand, for tensioning each strand. Since each pivoting lever is pivoted on its of pivot for pivoting about a stationary axis of pivot, the tensioning force of the tensioner is transmitted to the tension element and thus to the strand concerned, with a lever action corresponding to this ratio. This ratio determines whether the tensioning force of the tensioner is decreased or in creased by the pivoting lever before transmission to the tension element. The different specific ratios of the lengths of the pivoting levers form a common system of levers in the tensioning de vice. This lever system changes its leverage in keeping with the reaction force s transmitted from one strand to the other. The advantage of such a tensioning device is particularly that
the tensioning fores can be differently defined from strand to strand,
a variation of the tensioning forces is possible by replacing the pivoting levers with pivoting levers having a different length ratio,
only one tensioner-damper unit is required,
tensioning devices having different tensioning and damping characteristics can be designed by any suitable combination of tensioner-damper units and pivoting levers and
the traction element is pre-tensioned in accordance to its mode of operation only so far as required by the power flow and is thus freed of unnecessary wear.